1. Field of the Invention
The invention relates to apparatus for determining the concentration of toner particles in electrographic developer mixtures. More specifically, this invention relates to toner concentration monitoring apparatus of the type wherein toner concentration is determined by sensing the reflectivity of the developer mixture. Fresh toner is added when needed to maintain the toner concentration at a desired level.
2. Description of the Prior Art
In the electrographic reproduction process, the surface of a radiation-sensitive photoconductive member which may comprise a layer of photoconductive material disposed on a conductive backing, is given a uniform electrostatic charge and is then image-wise exposed to a pattern of actinic radiation corresponding to the indicia on a document or the like being reproduced. Such exposure serves to selectively dissipate the uniform charge on the surface, leaving behind a latent electrostatic image which can then be developed by contacting it with an electrographic developer mixture.
In general, electrographic developer mixtures comprise a mixture of suitably pigmented or dyed resin-based electroscopic particles, known as toner, and a granular carrier material which functions to carry such toner by generating triboelectric charges thereon. The development of the latent electrostatic image occurs when the developer mixture is brought into contact with the electrostatic image-bearing surface. Such contact is commonly effected by either cascading the mixture over such surface or brushing the surface with one or more rotating magnetic development brushes, the "bristles" of which comprise chain-like arrays of toner-carrying carrier particles. Upon contacting the electrostatic image-bearing surface, the toner particles, being charged to a polarity opposite to that of the electrostatic image, are separated from the carrier particles and are selectively deposited in an imagewise configuration on the image-bearing surface to form a developed image which may thereafter be transferred to a paper receiving sheet and fixed thereto by any suitable means, such as heat, to form a copy of the original document. As successive toner images are formed, toner particles are depleted from the developer mixture, requiring subsequent replenishment to avoid a gradual reduction in image density.
To avoid the necessity of periodic manual replenishment and the operating difficulties often encountered as a result of over-replenishment, a variety of devices has been heretofore proposed for automatically replenishing toner particles. Exemplary of photoelectric or optical toner concentration monitoring devices is the device disclosed in U.S. Pat. No. 3,233,781 issued to W. J. Grubbs which utilizes the reflectivity of a developer mixture having toner and carrier particles as a means for monitoring the concentration of toner particles in the developer mixture. Toner particles, usually being black, possess light absorbing surfaces and reflect less radiant energy than the carrier particles. Thus, the reflectivity of the developer mixture depends upon the relative proportions of the mixed particles. According to the Grubbs disclosure, the reflectance of the developer mixture is monitored by directing light from an incandescent lamp toward the developer mixture and detecting the light reflected by the developer mixture with a photocell. This photocell along with a similar photocell which is illuminated directly by the lamp are employed as variable resistance arms of a bridge circuit. The bridge circuit activates a toner replenishing device in response to a predetermined change in the ratio of photocell outputs, such change being characterized by an unbalance in the circuit.
While operable, photoelectric toner monitoring devices of the type described above have not proven entirely satisfactory in operation, especially over extended periods of time.
A significant advance is disclosed in commonly assigned U.S. Pat. No. 3,876,106 to Rowell et al issued Apr. 8, 1975 which discloses an apparatus that overcomes many of the above-described prior art problems. In the Rowell apparatus a light emitting diode (LED) illuminates a developer mixture of toner and carrier particles. The apparatus includes first and second photo-sensitive means, the first being disposed to receive radiation from the LED reflected from the developer mixture and the second being disposed to receive radiation directly from the LED. The first and second photosensitive means produce first and second analog signals representative of the reflectivity of the developer mixture and the intensity of the radiation of the LED, respectively. The first and second analog signals are then converted to digital signals.
This apparatus further includes a digital processing apparatus having a programmable digital computer with a stored program which in response to such first and second digital signals produces in accordance with such stored program a representation of the toner concentration of the developer mixture. Further, the apparatus from time to time adds fresh toner to the developer mixture to maintain a substantially constant developer mixture reflectance.
It has been observed that with usage of a developer mixture, the reflectivity of the developer mixture can not be faithfully used to determine toner concentration of the developer mixture. It is believed that the change in developer mixture reflectivity can be directly attributed to carrier particle scumming. By scumming it is meant that as the developer mixture is used or ages, the mechanical friction between the toner and carrier particles causes a film of toner to be coated on the outer surface of the carrier particles. As carrier particle scumming increases, the reflectivity of the carrier particles and consequently the developer mixture decreases. Thus prior apparatus may determine, based solely upon developer mixture reflectivity, that the toner concentration is higher than it actually is.